ntsync: add PROTON_NO_KERNEL_NTSYNC env switch to force userspace backend

.github/workflows/build-proton.yml

@@ -331,7 +331,7 @@ jobs:
             Single SDK 28 build with 16KB page size support (works on Android 9+ and Android 15+).
 
             ### Features
-            - Runtime ntsync backend selection: kernel `/dev/ntsync` when present and usable (probed by wineserver at startup), otherwise experimental userspace ntsync via [ntsync-android](https://github.com/GameNative/ntsync-android) (commit [`7ce6435`](https://github.com/GameNative/ntsync-android/commit/7ce6435e5979b1cb5341aa4b299f31e8937fe121)) — plus fsync and wineserver-side `ntsync_event_set` support; `PROTON_NO_NTSYNC=1` disables ntsync entirely
+            - Runtime ntsync backend selection: kernel `/dev/ntsync` when present and usable (probed by wineserver at startup), otherwise experimental userspace ntsync via [ntsync-android](https://github.com/GameNative/ntsync-android) (commit [`7ce6435`](https://github.com/GameNative/ntsync-android/commit/7ce6435e5979b1cb5341aa4b299f31e8937fe121)) — plus fsync and wineserver-side `ntsync_event_set` support; `PROTON_NO_NTSYNC=1` disables ntsync entirely, `PROTON_NO_KERNEL_NTSYNC=1` forces the userspace backend
             - `WINE_FAST_YIELD` fast-yield hook in `NtYieldExecution`
             - FEX integration (stats shm, `SkipThreadAttach` detach guard) via bylaws' arm64ec patches
             - FEX unixlib loader: `MemoryWineLoadUnixLibByName` / wow64 / unload support with `$PREFIX/lib/wine` fallback
@@ -340,8 +340,8 @@ jobs:
             - Stripped, `-g0 -O2` build: smaller tree, faster install (zstd-packed `.wcp`)
 
             ### ntsync changes in this build
-            - **Runtime kernel/userspace backend detection (proton-wine side)** — ntsync is no longer hard-selected at compile time. wineserver probes `/dev/ntsync` at startup with a real `NTSYNC_IOC_CREATE_EVENT` ioctl (catches SELinux/seccomp-blocked nodes): if the kernel driver is present and usable it is used directly; otherwise the build falls back to the userspace ntsync-android shm backend. Backend is chosen once per session and applies to all processes.
-            - **No shm region on kernel-ntsync devices** — the dead-process sweep is now gated on the userspace backend being active, so the ~960 KB shm file is never initialized when the kernel driver handles ntsync.
+            - **`PROTON_NO_KERNEL_NTSYNC=1` forces the userspace backend** — new runtime escape hatch for A/B testing: wineserver skips `/dev/ntsync` even when the kernel driver is present and working, and initializes the userspace ntsync-android shm backend instead. The forced backend is identifiable in logs: `ntsync: PROTON_NO_KERNEL_NTSYNC set, using userspace ntsync.`
+            - Runtime kernel/userspace backend detection (probed `NTSYNC_IOC_CREATE_EVENT`, userspace fallback, no shm region on kernel-ntsync devices) shipped in the previous build and is unchanged.
             - ntsync-android library is unchanged since the last build ([`7ce6435`](https://github.com/GameNative/ntsync-android/commit/7ce6435e5979b1cb5341aa4b299f31e8937fe121)); `PROTON_NO_NTSYNC=1` still disables both backends.
 
             ### Android / bionic fixes

android/ntsync_android/README.md

@@ -38,15 +38,19 @@ wineserver probes for in-process sync support once at startup
 
 1. `PROTON_NO_NTSYNC=1` disables ntsync entirely (both kernel and
    userspace); wineserver falls back to server-side synchronization.
-2. Otherwise it opens `/dev/ntsync` and **probes** it with a real
+2. `PROTON_NO_KERNEL_NTSYNC=1` skips the kernel driver even when it is
+   present and working, forcing the userspace backend
+   (`ntsync: PROTON_NO_KERNEL_NTSYNC set, using userspace ntsync.`).
+   Intended for A/B testing of the two backends.
+3. Otherwise it opens `/dev/ntsync` and **probes** it with a real
    `NTSYNC_IOC_CREATE_EVENT` ioctl (the node can exist but be unusable,
    e.g. SELinux policy or seccomp). If the probe succeeds, the kernel
    driver is used, exactly like upstream Proton.
-3. If the device is missing or the probe fails, it tries
+4. If the device is missing or the probe fails, it tries
    `ntsync_init()`; on success all processes attach to the shared-memory
    region and use the userspace implementation
    (`ntsync: no usable /dev/ntsync, using userspace ntsync.`).
-4. If that also fails, wineserver falls back to server-side
+5. If that also fails, wineserver falls back to server-side
    synchronization.
 
 wineserver tells each client which backend is in use through the

android/patches/common/server_inproc_sync_c.patch

@@ -1,5 +1,5 @@
 diff --git a/server/inproc_sync.c b/server/inproc_sync.c
-index a7020ef8fcd..c8691d1cd49 100644
+index a7020ef8fcd..6c6b4d4cc0f 100644
 --- a/server/inproc_sync.c
 +++ b/server/inproc_sync.c
 @@ -36,7 +36,14 @@
@@ -17,7 +17,7 @@ index a7020ef8fcd..c8691d1cd49 100644
 
  #ifdef NTSYNC_IOC_EVENT_READ
 
-@@ -45,6 +52,36 @@
+@@ -45,18 +52,89 @@
  #include <sys/stat.h>
  #include <unistd.h>
 
@@ -54,13 +54,24 @@ index a7020ef8fcd..c8691d1cd49 100644
  int get_inproc_device_fd(void)
  {
      static int fd = -2;
-@@ -53,10 +90,39 @@ int get_inproc_device_fd(void)
+     if (fd == -2)
+     {
++#ifdef __ANDROID__
++        /* escape hatch for A/B testing: PROTON_NO_KERNEL_NTSYNC=1 ignores a
++         * working kernel driver and forces the userspace backend */
++        int force_userspace = getenv( "PROTON_NO_KERNEL_NTSYNC" ) && atoi(getenv( "PROTON_NO_KERNEL_NTSYNC" ));
++#endif
          if (getenv( "PROTON_NO_NTSYNC" ) && atoi(getenv( "PROTON_NO_NTSYNC" )))
              fd = -1;
          else
 +        {
              fd = open( "/dev/ntsync", O_CLOEXEC | O_RDONLY );
 +#ifdef __ANDROID__
++            if (force_userspace && fd >= 0)
++            {
++                close( fd );
++                fd = -1;
++            }
 +            if (fd >= 0)
 +            {
 +                /* the device node can exist but be unusable (SELinux policy,
@@ -88,13 +99,15 @@ index a7020ef8fcd..c8691d1cd49 100644
              do_fsync_cached = 0;
 +#ifdef __ANDROID__
 +            if (ntsync_userspace)
-+                fprintf( stderr, "ntsync: no usable /dev/ntsync, using userspace ntsync.\n" );
++                fprintf( stderr, force_userspace
++                                 ? "ntsync: PROTON_NO_KERNEL_NTSYNC set, using userspace ntsync.\n"
++                                 : "ntsync: no usable /dev/ntsync, using userspace ntsync.\n" );
 +            else
 +#endif
              fprintf( stderr, "ntsync: up and running.\n" );
          }
          else if (do_fsync()) fd = FSYNC_USED_BY_SERVER;
-@@ -124,6 +190,10 @@ struct inproc_sync *create_inproc_internal_sync( int manual, int signaled )
+@@ -124,6 +202,10 @@ struct inproc_sync *create_inproc_internal_sync( int manual, int signaled )
      else
      {
          event->type = INPROC_SYNC_INTERNAL;
@@ -105,7 +118,7 @@ index a7020ef8fcd..c8691d1cd49 100644
          event->fd   = ioctl( get_inproc_device_fd(), NTSYNC_IOC_CREATE_EVENT, &args );
      }
      list_init( &event->entry );
-@@ -151,6 +221,10 @@ struct inproc_sync *create_inproc_event_sync( int manual, int signaled )
+@@ -151,6 +233,10 @@ struct inproc_sync *create_inproc_event_sync( int manual, int signaled )
      else
      {
          event->type = INPROC_SYNC_EVENT;
@@ -116,7 +129,7 @@ index a7020ef8fcd..c8691d1cd49 100644
          event->fd   = ioctl( get_inproc_device_fd(), NTSYNC_IOC_CREATE_EVENT, &args );
      }
      list_init( &event->entry );
-@@ -178,6 +252,10 @@ struct inproc_sync *create_inproc_mutex_sync( thread_id_t owner, unsigned int co
+@@ -178,6 +264,10 @@ struct inproc_sync *create_inproc_mutex_sync( thread_id_t owner, unsigned int co
      else
      {
          mutex->type = INPROC_SYNC_MUTEX;
@@ -127,7 +140,7 @@ index a7020ef8fcd..c8691d1cd49 100644
          mutex->fd   = ioctl( get_inproc_device_fd(), NTSYNC_IOC_CREATE_MUTEX, &args );
      }
      list_add_tail( &inproc_mutexes, &mutex->entry );
-@@ -206,6 +284,10 @@ struct inproc_sync *create_inproc_semaphore_sync( unsigned int initial, unsigned
+@@ -206,6 +296,10 @@ struct inproc_sync *create_inproc_semaphore_sync( unsigned int initial, unsigned
      else
      {
          sem->type = INPROC_SYNC_SEMAPHORE;
@@ -138,7 +151,7 @@ index a7020ef8fcd..c8691d1cd49 100644
          sem->fd   = ioctl( get_inproc_device_fd(), NTSYNC_IOC_CREATE_SEM, &args );
      }
      list_init( &sem->entry );
-@@ -228,17 +310,23 @@ static void inproc_sync_dump( struct object *obj, int verbose )
+@@ -228,17 +322,23 @@ static void inproc_sync_dump( struct object *obj, int verbose )
 
  void signal_inproc_sync( struct inproc_sync *sync )
  {
@@ -164,7 +177,7 @@ index a7020ef8fcd..c8691d1cd49 100644
      else            ioctl( sync->fd, NTSYNC_IOC_EVENT_RESET, &count );
  }
 
-@@ -262,6 +350,9 @@ static void inproc_sync_destroy( struct object *obj )
+@@ -262,6 +362,9 @@ static void inproc_sync_destroy( struct object *obj )
      assert( obj->ops == &inproc_sync_ops );
      list_remove( &sync->entry );
      if (do_fsync()) fsync_free_shm_idx( sync->fd );
@@ -174,7 +187,7 @@ index a7020ef8fcd..c8691d1cd49 100644
      else            close( sync->fd );
  }
 
-@@ -277,7 +368,13 @@ void abandon_inproc_mutexes( thread_id_t tid )
+@@ -277,7 +380,13 @@ void abandon_inproc_mutexes( thread_id_t tid )
      }
 
      LIST_FOR_EACH_ENTRY( mutex, &inproc_mutexes, struct inproc_sync, entry )
@@ -188,7 +201,7 @@ index a7020ef8fcd..c8691d1cd49 100644
  }
 
  static int get_obj_inproc_sync( struct object *obj, int *type )
-@@ -359,6 +456,10 @@ DECL_HANDLER(get_inproc_sync_fd)
+@@ -359,6 +468,10 @@ DECL_HANDLER(get_inproc_sync_fd)
 
      if ((fd = get_obj_inproc_sync( obj, &reply->type )) < 0) set_error( STATUS_NOT_IMPLEMENTED );
      else if (do_fsync()) reply->fsync_shm_idx = fsync_grab_shm_idx( fd );